Port 25: SMTP
Mail transfer between mail servers. Must be open inbound on a mail server. Outbound port 25 from VMs is how spam leaves hosting networks, so many providers block it by default.
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Port 25 at a glance
Port25
ProtocolTCP
ServiceSMTP
Open to the Internet?Yes, if you run this service publicly
See What Is Listeningss -lntup | grep ':25 '
Test From Outsidenc -zv your-server 25
Block with UFWufw deny 25/tcp
Block with nftablesnft add rule inet filter input tcp dport 25 drop
SMTP uses TCP port 25. Mail transfer between mail servers.
Must be open inbound on a mail server. Outbound port 25 from VMs is how spam leaves hosting networks, so many providers block it by default.
Open port 25 only on servers that run SMTP, and keep the software patched: public services are scanned within minutes of going online.
Other Ports
20 FTP Data21 FTP22 SSH23 Telnet25 SMTP53 DNS67 DHCP Server80 HTTP110 POP3123 NTP143 IMAP161 SNMP389 LDAP443 HTTPS445 SMB465 SMTPS514 Syslog587 Mail Submission853 DNS over TLS993 IMAPS995 POP3S1194 OpenVPN1433 Microsoft SQL Server1900 SSDP / UPnP2049 NFS2083 cPanel (SSL)2087 WHM (SSL)3306 MySQL / MariaDB3389 RDP3478 STUN / TURN5060 SIP5432 PostgreSQL5900 VNC6379 Redis8006 Proxmox VE8080 HTTP Alternate8443 HTTPS Alternate11211 Memcached25565 Minecraft27017 MongoDB51820 WireGuard
Questions
What is port 25 used for?
SMTP. Mail transfer between mail servers.
Is it safe to open port 25?
Yes, if you run this service publicly. Must be open inbound on a mail server. Outbound port 25 from VMs is how spam leaves hosting networks, so many providers block it by default.
How do I check if port 25 is open?
On the server, ss -lntup | grep ':25 ' shows what is listening. From another machine, nc -zv your-server 25 shows whether the firewall lets it through.
How do I block port 25?
With UFW: ufw deny 25/tcp. With nftables, add a drop rule for dport 25 in your input chain.
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